This invention relates to control units for air distribution systems and, more particularly, to damper arrangements for use in terminal control units for air distribution systems.
It is well known that air flow terminal units for air distribution systems are subject to self-generated noise which may reach excessive levels, particularly when air is being delivered at relatively high velocities and pressures. Generally, it is necessary to provide some sort of noise attenuation such as by lining the interior of the unit with a sound-absorbing material. Conventional porous materials used for this purpose are capable of absorbing sound waves having lengths up to four times the thickness of the material. However, excessive thickness of the material are required to attenuate low frequency sound which has relatively long waves. For example, some low frequency sound waves can be as long as 48 inches and require a 12 inch layer of sound-absorbing material. The size of the housing required to accommodate a material of this thickness obviates this approach as a practical solution for many installations.
Prior art constructions have employed perforated plates, located upstream and/or downstream of the control damper, to break the mainstream into a plurality of smaller streams having a higher frequency sound which can be more easily attenuated. While alleviating the problem in some cases, such prior art constructions are not without disadvantages. The perforated plates can create excessive pressure drops and/or the divided air streams can subsequently merge back together before they have passed through the damper or the air outlet.
Exemplary prior art constructions of control arrangements which employ perforated members to minimize noise generation are disclosed in U.S. Pat. Nos. 3,911,958 (Logsdon), 3,750,839 (McNabney) and 2,807,329 (Caldwell).